A nano-sized membrane that looks like a hard, hairy sponge can detect minute amounts of lead and cadmium in water, report researchers who devised a new method in the laboratory to grow the natural material – called hydroxyapatite – that makes up the membrane.

Hydroxyapatite has unique properties that are already being exploited in a variety of sensors in an effort to move away from using gold, mercury and other toxic substances. The new method – which is still being researched – takes its use even further by developing crystal-like protrusions and structures that increase the material’s surface area. Its higher interaction with the surrounding water dramatically improves the material’s sensitivity; it can measure levels up to 100 times lower than other available devices.

Heavy metals such as lead and cadmium are a major health and environmental problem. They contaminate soil and water and accumulate in plants and animals. People are exposed to heavy metals through many sources, including food, water, air and dust. In humans, exposure to heavy metal is linked to many disorders such as organ damage, developmental problems and psychological changes.

Governments regulate heavy metal emissions, but the metals still contaminate many streams, lakes and other water bodies. As regulation tighten, it is thus important to monitor for these pollutants at more sensitive levels in waterways, tap water and drinking water.

The researchers grew the hydroxyapatite – which is a major component of human teeth and bones – on a membrame for four to seven days to create a crystal-like material with a high surface area. A higher surface area allows for increased surface-water interaction.

The hydroxyapatite can efficiently exchange calcium from its own structure with lead and cadmium in the water. The metal content in the water is then determined by applying a varying voltage over the hydroxyapatite and measuring the electrical current that flows through it.

The detection limit for lead was determined to be 9 X 10-10 grams per liter (g/L) – that is, a decimal point followed by nine zeros and a nine. Cadmium was detected down to 3 X 10-9 g/L. These detection limits are on the order of parts per trillion. They could detect a single drop in the volume of 20 Oympic size swimming pools. This makes the innovative material 10 to 100 times more sensitive than other hydroxyapatite-based sensors.

The high surface area hydroxyapatite was formed by controlling its growth using a Nafion membrane. Nafion is a polymer that allows only positive ions to pass through it. Hydroxyapatite was formed by combining two reactants – calcium and phosphate – added on opposite sides of the membrane in water. Only calcium – with its positive charge – can pass through the membrane The hydroxyapatite forms on the membrane’s surface as calcium reaches the other side and combines with the water and phosphate.

The new method to prepare high surface area hydroxyapatite is very promising. However, applying it to detect heavy metals will take further work to optimise. For example, the authors only tested their device to detect a single metal. They did not yet report the effectiveness when several metals are present or when other non-metallic pollutants are present in the water.

The researchers will also need to optimise how to form the high surface area hydroxyapatite. For wide use, faster preparation methods will need to be developed.